The invention concerns a magnet system for nuclear magnetic resonance (NMR) tomography for the production of an approximately at least 8th order homogeneous static magnetic field within an investigational volume having magnetic coils (S1, S2) which are coaxial to a z axis running through the center of the investigational volume and arranged on both sides of the investigational volume and which, relative to the center of the investigational volume (z=0), assume z positions z.sub.S1 and z.sub.S2 respectively, and exhibit average separations r.sub.S1 and r.sub.S2 respectively from the z axis and, during operation, have DC currents flowing through them in the same direction.
A magnet system of this kind is, for example, known in the art from DE 39 07 927 A1.
NMR tomography systems have their main medical application in the area of whole body tomography for non-invasive investigations of patients. Towards this end, on the one hand, when configuring the system, one must take care that the homogeneity of the static magnetic field is as high as possible so that a high resolution of the slice images created can be achieved and, on the other hand, the treating physician should have ample spatial access to the patient being examined so that, for example, a concurrent treatment of the patient simultaneous with slice image generation is facilitated. Furthermore for the purpose of preventing claustrophobia on the part of the patient, it is desirable when the structure of a whole body tomograph of this kind is as "open" as possible.
An electromagnetic system for nuclear spin tomography is described in DE 36 16 078 C2 with which a resistive H-magnet with suitable pole shoes is utilized. Although this system known in the art allows the possibility of transverse access to the investigational volume, there is however, no free axial access thereto. Due to the massive pole shoes there are, in general, significant eddy current problems. The homogeneous magnetic field is largely produced not by the geometry of the coils rather through the shape of the pole shoes which significantly limits the variational possibilities in the configuration of the magnet fields. In contrast to the so-called "air coil configurations", to which the above cited magnet system according to DE 39 07 927 A1 belongs, the pole shoe electromagnetic systems in accordance with DE 36 16 078 C2 are a completely different magnet type which exhibits completely different problems associated with its characteristics.
A magnetic coil configuration is known in the art from EP 0 011 335 B1 which belongs to the "aircoil" magnet type. Here one is dealing with a double Helmholtz configuration having at least four individual current guiding coils with which the currents flow during operation in parallel through the coils. This configuration known in the art allows only an extremely narrow gap for sideward access due to the geometry of the magnet coils which can, for practical purposes, not be utilized by a therapist.
In the coil configuration known in the art from DE 32 45 944 C2 one is likewise dealing with a double Helmholtz configuration having at least four individual current carrying coils with which the currents flow in parallel through the coils during operation. This configuration, which is known in the art does not, however, allow for a transverse access due to the iron shielding provided thereby, since the shielding is completely closed. Even if the iron shielding were eliminated, only a theoretical transverse access similar to the configuration in accordance with the above mentioned EP 0 011 335 B1 is possible.
In contrast thereto the above cited DE 39 07 927 A1 describes an air coil system having a double Helmholtz configuration with which the four partial coils provided for are fed with oppositely directed currents to facilitate a substantially larger separation between the coil pairs in the axial direction so that a relatively large transverse access to the investigational volume from the side is given. This configuration would facilitate a direct treatment of the patient being examined on the part of a therapist, in particular an operator, during which time the effectiveness of the treatment can be monitored through the simultaneous transfer of the recorded tomograms to a monitor at a visible distance from the therapist. A disadvantage of the magnet system according to DE 39 07 927 A1 is, however, that the coil configuration is relatively complex and that significantly higher ampere current numbers are necessary, due to the oppositely directed current feed of the coils for the production of the magnetic field, compared to the above mentioned double Helmholtz configuration having parallel current feed as is, for example, known in the art from EP 0 011 335 B1 or DE 32 45 944 C2. In particular, with resistive magnet coils, significantly higher electrical power is thereby necessary for producing the magnetic field.
U.S. Pat. No. 4,710,741 discloses a coil arrangement for producing a magnetic field of high homogeneity comprising a single pair of identical annular coils arranged coaxially in spaced relationship as well as a pair of ferromagnetic rings disposed coaxially to the coils, and symmetrically with respect to a plane perpendicularly intersecting the axis of the coils, half-way between them. The coils and ferromagnetic rings are dimensioned and positioned to eliminate the more significant spherical harmonic coefficients with the coils being powered with equal energizing currents.
GB 2219406A discloses an electromagnet having an x and y axis. A first and a second coil, having a plurality of windings, are arranged on opposite sides of the y axis as are a first and a second ferromagnetic ring. The first and the second coils and the first and the second rings are axially spaced apart along the direction of the x axis and are symmetrically arranged relative to the x and y axes. A cylindrical region for containing a shaped magnetic field (e.g. substantially spheroidal or substantially ellipsoidal) produced by means of the coils and rings is located within the electromagnet, the field having a predetermined amount of homogeneity in the cylindrical region.
In view of the above prior art, it is the purpose of the present invention to present a magnet system of the above mentioned kind with which a particularly good transverse access to the investigational volume is facilitated, whereby the coil configuration is simplified compared to the configuration known in the art, and a reduced ampere winding number for the production of a homogeneous magnetic field of the same field strength is required or reduced electrical power and thereby reduced requirements for the cooling of the configuration.